1. Field of the Invention
The present invention concerns a device for regenerating a wavelength-division multiplex (WDM) optical signal. It applies in particular to fiber optic systems for transmitting binary data.
2. Description of the Prior Art
An optical signal which propagates in optical fibers, in communication nodes and other optical devices used in telecommunications, is subject to optical losses and is modified. It would therefore seem necessary to regenerate the signal to compensate for accumulated unwanted noise, distortion of the signal and time shifting.
The regenerators currently available, whether of the opto-electronic or all-optical type, cannot easily regenerate a multiplex signal in parallel on all the WDM channels.
Opto-electronic regenerators available off the shelf comprise an electronic conversionxe2x80x94electronic processingxe2x80x94optical conversion system. This type of device detects the optical signal before processing it in the electronic domain. This electronic processing of the signal compensates for distortion of the signal and effects what is known as xe2x80x9c2Rxe2x80x9d regeneration (Reshaping, Resynchronization). The regenerated signal can then be transmitted by a laser which amplifies the signal. This achieves what is called xe2x80x9c3Rxe2x80x9d regeneration (Reshaping, Resynchronization, Re-amplification).
However, this type of device cannot be used directly to regenerate a WDM signal because it detects the total luminous power of the signal, i.e. the power of all the WDM channels.
Consequently, in order to be able to process all the channels, they must first be wavelength-division demultiplexed. In this case, regenerating a wavelength-division multiplex signal therefore entails using an opto-electronic regenerator for each WDM channel. It is therefore necessary to use as many regenerators as there are WDM channels.
Other all-optical regenerator devices have been designed. These devices are guided wave optical circuits. They include a waveguide stripe having non-linear properties and into which the light signal to be regenerated, which comes from an optical fiber, must be injected. To achieve this, the waveguide stripe and the optical fiber must be aligned. This solution requires as many fiber splices as there are WDM channels. For example, if the optical signal to be regenerated comprises 16 WDM channels, either the same number of dies each having a waveguide stripe or an optical integrated circuit comprising 16 waveguide stripes is required for the 16 channels of the optical signal to be regenerated. In either case, this solution is much too costly.
The invention overcomes the aforementioned drawbacks in that it proposes an all-optical regenerator that is not based on the guided wave principle and avoids the problem of multiple fiber splices according to the number of channels. Moreover, by virtue of the invention, a single regenerator can be used to regenerate the various beams from the various WDM channels on the optical fiber. Another aim of the invention is for the regenerator to be able to regenerate WDM channels carried by a greater number of wavelengths that are not predefined. The invention avoids the use of a regenerator for each WDM channel and circumvents synchronization constraints on signal processing.
The invention relates more particularly to a device for regenerating a wavelength-division multiplex optical signal, which device comprises:
a dispersive medium for receiving the wavelength-division multiplex signal and emitting a corresponding dispersed wave into a free space, and
a saturable absorber disposed to receive the dispersed wave on a first face and to transmit a corresponding regenerated wave.
The device of the invention can regenerate the multiplex optical signal, and in particular reshape it, for all the WDM channels simultaneously and with no synchronization constraints. The wave from an optical fiber is focused onto a saturable absorber plate at points which differ according to the wavelengths of the channels, because of the dispersive medium of the device. The device of the invention can therefore separate and regenerate the WDM channels over all of a continuous band of wavelengths.
According to another feature of the invention, the device further comprises another dispersive medium for recombining the dispersed and regenerated wave at the exit from the saturable absorber.
The invention also concerns a saturable absorber which can be used in the regenerator device. Saturable absorbers available off the shelf are made by two different methods.
A first method entails growing the active layer, i.e. the absorbent layer, at low temperature. The active layer is generally made from a ternary material, for example AlGaAs or InGaAs, and includes quantum wells. However, aggregates form in the material during growth at low temperature and degrade the excitonic line. The excitonic line can become sufficiently degraded to prevent recombination of the free carriers. This low temperature growth method requires additional doping with Be (Beryllium) to prevent excessive degradation of the excitonic line, but this doping increases the cost of the saturable absorber.
A second method entails ionic irradiation of the absorbent layer to introduce recombination centers and to enable the carriers created by the photons to recombine very quickly. Ionic implantation reduces the lifetime xcfx84 of the carriers, i.e. increases the speed of recombination. However, ionic irradiation also tends to widen the excitonic line, which reduces the recombination yield. A rate of ionic implementation must therefore be found to achieve a compromise between a sufficiently low carrier lifetime xcfx84, in the order of one picosecond (ps), and a reasonable combination yield, i.e. few residual losses.
What is more, in conventional saturable absorbers, the absorbing layer is relatively thick. Its thickness is in the range from 2 xcexcm to 5 xcexcm. Growing this layer with multiple quantum wells epitaxially to this thickness also contributes to increasing the manufacturing cost of the saturable absorbers.
To avoid all the above problems, the invention proposes two embodiments of a saturable absorber.
The first embodiment entails controlled introduction of dislocations into the crystal structure of the active layer, to create artificial recombination centers. This eliminates the need for ionic irradiation, which is a complex and costly technique. To this end, a material having a lattice mismatch with the material of the protection layer, i.e. InP in the example given here, is grown on top of the absorbing layer, which is covered with a protective layer of InP, for example. The material grown on the InP is gallium arsenide (GaAs), for example. Growing a material of this kind (GaAs) having a lattice mismatch with the InP creates tensions at the interface between the two materials. The tensions cause the appearance of dislocations in the InP protective layer which propagate into the active layer.
The second embodiment, which can complement the first one, consists in reducing the thickness of the active layer to reduce the cost of manufacturing the saturable absorber. To this end, it entails placing two mirrors on respective opposite sides of and parallel to the active layer. The light wave passing through the active layer therefore undergoes multiple reflections and is therefore absorbed several times. The thickness of the active layer can therefore be reduced by a factor corresponding to the number of reflections.